The invention is based on a known technique, which will here be described briefly herein below.
On the market there is a device, a so-called scanner, intended for detecting flying objects by means of IR-rays emitted from the objects. Such a scanner comprises a sensor unit, an evaluation unit and a display and control unit. The sensor unit receives IR-rays from the objects to be detected within the momentarily scanned solid angle. The sensor unit transmits the corresponding signals to the evaluation unit, which processes the signals and upon the significant detection of objects, i.e., in a military context, targets, transmits the corresponding signals including direction coordinates to the display and control unit. From it the collected output data go, which in a military context partly takes the form of a visible picture on a screen, and partly the form of signals, may be forwarded online and e.g., be used for guidIng a fire-control system for anti-aircraft defense.
The evaluation unit has as its purpose to select those signals, among the signals received from the sensor unit, which are significant, i.e., which indicate targets within the scanning range of the sensor unit, and to indicate when such targets appear, and generally to indicate their coordinates. The evaluation unit functions thus according to preselected programmed criteria of what will be considered significant objects, i.e., targets.
The evaluation unit has a filter function and a decision function. The signals which are received from the sensor unit (measured intensity as a function of direction) are inputted to the filter which is designed to enhance the typical signals of targets. During filtering the directional information is retained. A resulting filter output signal for a certain direction is a measure of the probability that there is a target in the actual direction.
A concrete example is a filter, which for every direction forms the difference between measured intensity in the actual direction and the average intensity in a two-dimensional interval of the surrounding directions. Typical for a filter in this application is particularly that the output signal for a certain direction is a weighted sum of the input signals of the filter in an angular range in and about that direction.
In a scanner whose purpose is to select significant objects, i.e., targets, in addition to the filter, a decision function is also required. That function decides whether a significant object exists or not. The most common decision function is to compare the output signal of the filter with a threshold level. If the threshold is exceeded, a significant object, i.e., a target, is indicated. The mechanism which chooses the threshold level may also be included in the decision function. The threshold level is often determined through statistical evaluation of the output signals of the filter within a large range, possibly the whole scanning range. The object is to find a level which is exceeded at an acceptably low frequency in the absence of significant objects, i.e., targets (false-alarm frequency) and which yet is not too high for appearing targets to be indicated with certainty.
Scanners of the type described aboved function preferably within the IR-spectral ranges 3 to 5 and 7 to 13 micrometers, respectively, which represent "windows" with regard to the transmission spectra of the atmosphere for IR-radiation. That means that the focusing means of the sensor unit, which in itself can consist of a lens or mirror, usually consists of a silicon lens for the 3 to 5 micrometer range and a germanium lens for the 7 to 13 micrometer range, i.e., it is chosen with respect to the actual spectral range. In view of hitherto existing corresponding radiation-sensitive detector elements, such a lens must be made comparatively large, in order that the sensor unit will produce signals such that the evaluation unit can detect significant targets with any appreciable precision.
Such a scanner cannot distinguish between birds (i.e., insignificant objects) at a relatively close distance and aircraft (i.e., significant objects) at a longer distance) which means that birds could cause a false alarm which is a great disadvantage in hitherto known scanners of the type described above.
It is therefore an object of the present invention is to design a device wherein it is possible to distinguish between relatively close objects from which radiation is emitted, and such objects which are located at a longer distance, but which objects when observed subtend a similar solid angle.